1. Field of Invention
The present invention relates to semiconductor chip packages, such as quad flat no-lead (QFN) packages and, more specifically, to a system and method for reducing pitch and increasing the lead count on such packages.
2. Description of Prior Art
An integrated chip package, such as a QFN package, comprises a plastic encapsulated semiconductor die mounted to a lead frame having a die pad and exposed outer leads on the periphery of the package. The lead frame is commonly fabricated from an electrically conductive material, such as metal, and chemically etched according to a predetermined pattern to form the leads. The pattern of lead is dictated by the use of an etching mask that is positioned against the lead frame and dictates which portions are etched away. Once the lead frame has been etched to form the leads, excess material is removed by saw singulation.
Electronic devices using such packages are constantly shrinking in size while demanding increased processing power. As faster and more powerful chips are developed, however, the I/O counts increase and a higher number of leads are required on the package. The numbers of leads that can be formed by conventional etching, however, are limited by the thickness of the lead frame material and the physical requirements for a minimum degree of spacing between leads. Thus, there is a constant need to increase the number of leads in a QFN package while reducing the space used by the leads.
One attempt to increase the number of leads on the lead frame involves the etching of double rows of leads into periphery of the lead frame. The use of additional rows, however, increases the overall footprint of the package and requires an additional saw singulation step to free the additional row from the lead frame. Previous attempts to increase the number of leads in other types of packages have involved the bonding of an additional row of leads over the top of the primary leads with an insulating adhesive. The physical bonding of additional rows, however, requires additional processing steps and may increase the overall height of the package.
3. Objects and Advantages
It is a principal object and advantage of the present invention to provide a chip package having a high number of leads that does not increase the dimensions of the package.
It is an additional object and advantage of the present invention to provide a method for forming a chip package having a high number of leads that does not require additional processing steps.
It is a further object and advantage of the present invention to provide a chip package having a high number of leads that can be fashioned from a single lead frame.
Other objects and advantages of the present invention will in part be obvious, and in part appear hereinafter.
In accordance with the foregoing objects and advantages, the present invention comprises a chip package having an array of leads that are staggered in all three dimensions, thus increasing the number of leads available in a confined space while maintaining the minimum separation necessary between adjacent leads. The leads are formed by over etching the lead frame with asymmetric top and bottom masks that are offset from one another. Although the resulting leads are staggered in three dimensions, no additional processing steps are needed beyond those used to fabricate conventional packages.
In one embodiment, the present invention contemplates a lead frame for use in a chip carrier package, comprising a central region having a first plurality of sides; a peripheral edge region having a second plurality of sides positioned in spaced relation around the central region; a first set of leads extending from each of the second plurality of sides, each of the first set of leads being partially defined by a first terminal end, first opposing side surfaces each of which extends along a respective first longitudinal axis, and a lower surface that extends in a first plane; a second set of leads extending from the second plurality of sides, each of the second set of leads being partially defined by a second terminal end, second opposing side surfaces each of which extends along a respective second longitudinal axis, and an upper surface that extends in a second plane; wherein each of the first set of leads are positioned in staggered relation to corresponding ones of the second set of leads such that the first terminal ends are spaced from the second terminal ends by a first predetermined distance, each of the first longitudinal axes are parallel to and spaced from the adjacent ones of the second longitudinal axes by a second predetermined distance, and the first plane is spaced from the second plane by a third predetermined distance.
The preferred method for forming the lead frame of a predetermined thickness for a chip carrier package, comprises the basic steps of providing a lead frame having an upwardly facing surface and a downwardly facing surface, a central region having a first plurality of sides, and a peripheral edge region having a second plurality of sides extending around and spaced from the first plurality of sides; providing an upper mask to position resist material on the upwardly facing surface that define a first set of leads extending from each of the second plurality of sides, each of the first set of leads being partially defined by a first terminal end, first opposing side surfaces each of which extends along a respective first longitudinal axis, and a lower surface that extends in a first plane; providing a lower mask to position resist material on the downwardly facing surface that define a second set of leads extending from the second plurality of sides, each of which is partially defined by a second terminal end, second opposing side surfaces each of which extends along a respective second longitudinal axis, and an upper surface that extends in a second plane, and a portion of the second set of leads that are vertically aligned with a portion of the first set of leads; etching away portions of the upwardly facing surface that are not covered with the resist material to a depth greater than one half of the predetermined thickness; and etching away portions of the downwardly facing surface that are not covered with the resist material to a depth greater than one half of the predetermined thickness, wherein each of the first set of leads are positioned in staggered relation to corresponding ones of the second set of leads such that the first terminal ends are spaced from the second terminal ends by a first predetermined distance, each of the first longitudinal axes are parallel to and spaced from the adjacent ones of the second longitudinal axes by a second predetermined distance, and the first plane is spaced from the second plane by a third predetermined distance.
The chip package resulting from this manufacturing process comprises a first set of leads each of which is partially defined by a first terminal end, first opposing side surfaces each of which extends along a respective first longitudinal axis, and a lower surface that extends in a first plane, and a second set of leads each of which is partially defined by a second terminal end, second opposing side surfaces each of which extends along a respective second longitudinal axis, and an upper surface that extends in a second plane, wherein each of said first set of leads are positioned in staggered relation to corresponding ones of said second set of leads such that said first terminal ends are spaced from said second terminal ends by a first predetermined distance, each of said first longitudinal axes are parallel to and spaced from the adjacent ones of said second longitudinal axes by a second predetermined distance, and said first plane is spaced from said second plane by a third predetermined distance. A chip may then be mounted to the die pad and wire interconnects can interconnect the chip""s input/outputs to corresponding ones of the first and second sets of leads. An epoxy resin, or other conventional encapsulation material, may then encapsulate the package which may then be mounted on an integrated circuit board.